The present invention relates to a brake disc for a vehicle disc brake, comprising a radially inner hub portion intended to be non-rotatably joined to a wheel hub, a surrounding, radially outer disc portion with opposing frictional surfaces and passages formed in the outer disc portion, said passages being arranged to conduct cooling air in the disc material between the frictional surfaces.
So-called ventilated brake discs, which are made to increase the cooling effect of the ambient air by diverting heat generated during braking, occur in two principle types. One type has channels which extend radially through the disc from the inner periphery of the hub to the outer periphery of the surrounding disc portion. The channels thus have inlet openings in the inner periphery and outlet openings in the outer periphery. As the disc rotates, a guided airflow is created through the channels from the inlet to the outlet. The other type of ventilated brake disc consists of two disc elements, which are fixed to a hub and to each other side-by-side with an intermediate space created by a plurality of spacers in the form of pillars.
By ventilating the disc, the mean temperature of the disc is reduced. Since the wear on the disc and linings is strongly depending on temperature, a reduction in temperature will have a positive effect on this wear. Furthermore, the highest temperature of the disc is reduced for most driving styles, which in turn reduces the risk of brake fade and the risk of cracking. One disadvantage of having channels opening into the inner periphery of the hub is, however, that the material mass is reduced in that portion of the brake disc where the stresses are greatest, both as regards braking force and temperature difference. The high stresses in connection with the holes in the hub increase the risk of cracking. In the other type of brake disc with spacers in the form of pillars between two disc elements, one does not have the above mentioned negative stress profile, but on the other hand, no fan action is achieved either.